A discrete multi-tone (DMT) modulation transmission method is used in, for example, an access based metal line such as a very high bit rate digital subscriber line (VDSL). In recent years, as the traffic of a network has increased, application of the DMT modulation to an optical transmission technology has been researched and developed.
The DMT modulation is a multi-carrier transmission technology (see, e.g., Patent Documents 1 and 2) based on an orthogonal frequency division multiplexing (OFDM) technology. In the DMT modulation, data are allocated to a plurality of subcarriers (SCs) having different frequencies, respectively, and the allocated data are modulated based on a multi-level degree and signal power depending on a transmission characteristic for each SC to be transmitted as a DMT signal. Herein, examples of the multi-level modulation used for the data of each SC may include quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM).
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 2010-147983, 2003-332853, and 2002-223171.